This invention relates to resonant cavities for waveguide systems. More particularly, it concerns resonant multicavity optical circuits especially suited for removing selected wavelength-distinct channels of light energy propagated in an optical waveguide in a manner that minimizes loss of energy from other channels in the waveguide and for providing enlarged free spectral range in the passby function, that is, the ratio of the electric field passed by a channel-removing tap to the electric field approaching the tap.
In the above-noted parent application Ser. No. 625,543, several embodiments of resonant cavity filter devices are disclosed for tapping spectral line widths as narrow as 0.01 Angstrom from an optical waveguide or fiber propagating many relatively broad bandwidth signals modulating light energy in the 1 to 2 micron region of the electromagnetic spectrum. As pointed out in that application, a principal objective in the removal of very narrow line widths from an optical waveguide trunk is the potential for the realization of an optical communications system in which a large number of information channels may be transmitted along a single optical waveguide trunk with a facility for tapping discrete channels at successive taps or branch lines along the trunk. It is also recognized that the number of successive taps on a single trunk is limited principally by the removal of energy from side channels by upstream taps whose purpose is to remove only one channel.
Certain of the embodiments disclosed in the aforementioned co-pending application employ so-called serial resonant cavities tuned in what is termed a vernier or coresonant relationship so that only one channel is coresonant in each of the serial resonant cavities, thus reducing or eliminating energy removed from the side orders of the desired coresonant channel. The narrow line wavelengths of interest are passed to a first resonant cavity in which the line width of interest is resonated along with side orders of the line width of interest. The second resonant cavity is coupled to the first resonant cavity and tuned to include the line width of interest, but with side orders at different spectral intevals than those of the first resonant cavity so only the line width of interest will be coresonant in both cavities. In this manner, only the desired, coresonant channel will be available at the output of the second resonant cavity.
Although these serial cavities demonstrate great potential for the selective tapping of very narrow line widths of light energy from an optical waveguide, the approach is restricted by the light energy loss accumulating after successive taps from a trunk. That is, these resonant taps, while effective in removing selected wavelength-distinct channels, do reduce the transmission energy of the wavelengths that are side orders of the selected wavelength.
Additionally, the parent application also describes an embodiment which provide integral dual cavities in which one cavity is completed by a path through a second cavity. The latter arrangement not only provides the above-noted vernier effect to pass only the selected wavelength to the branch line, but also improves the passby function as subsequently explained in detail in the present application.